1. The Field of the Invention
The present invention relates to active reflector radio frequency distance measuring systems and, more particularly, to systems which operate in a frequency-hopping mode, and employ both an adaptive loop for oscillator synchronization within a reflector unit in response to random phase sampling and, optionally, a phase-coherent delta sigma phase lock loop for accurate signal phase detection and creation at multiple frequencies.
2. Background and Related Art
Ranging, or the measurement of distance, through the use of phase measurements on radio frequency signals transmitted between two points in space is a well-known method of determining distance between two points. Given that simultaneous transmission and reception on the same carrier frequency is not possible because of mutual interference, transmission of radio frequency signals on path AB between stations A and B requires the use of a different carrier frequency for the return on the path BA.
Thus, such a distance measuring system requires two simultaneously occupied transmission channels and transmitters and receivers on two different frequencies. Such two frequency systems make inefficient use of the available ratio frequency spectrum. In addition, noise on either frequency may interfere with the process.
Ranging effected by measuring the phase of signals sent from point A, to point B, and back to point A is a well-established technology. For example. U.S. Pat. No. 3,243,812 to Williams discloses a system of phase measurement for determining distance. U.S. Pat. No. 4,170,773 to Fitzsimmons, et al. also discloses a method for determining distance by comparing the phase of a transmitted signal with one transponded by a distant device.
The Williams method employs the following steps: transmitting a modulated carrier at frequency f1 from point A (the interrogator) to point B (the transponder); coherently recovering the modulation (called range tones) by means of a receiver at point B; impressing this modulation on another carrier of frequency f2, which is then transmitted from point B to point A, where the modulation is recovered by a receiver at point A. Two versions of the range tones are then simultaneously available at point A: the original tone transmitted to point B; and the tone received from point B. The range, or distance, from point A to point B is determined by measuring the relative phase of the transmitted tones relative to the received tones and computing the distance using the following equation:D=cφ/2ωm−d where                D is the distance from point A to point B,        c is the velocity of light,        φ is the measured phase in radians,        ωm is the angular frequency of the modulation in radians/second,        d is the effective distance of the delay through the transmitting and receiving hardware, and        the integer, 2, in the denominator of the equation takes into consideration the transponded, double-distance path.        
Because phase measurements are ambiguous for modulus 2π, the corresponding distance will be ambiguous for modulus cπ/ωm. However, ambiguity can be resolved by taking measurements on multiple tones at either lower frequencies or at low difference frequencies.
Such prior art systems require transmission and reception simultaneously on two different frequencies. Phase measurements are made only at the interrogator using a single phase reference source, and the interrogator and receiver hardware perform different functions. The interrogator contains the source of the signal sent around the loop from interrogator to transponder and back, as well as the measurement apparatus for determining the relative phase between the transmitted signal and the received signal. The transponder functions merely to receive the ranging signal and to retransmit it with minimum, but known, delay or delay variation.
One limitation on the use of such prior art ranging systems is the requirement for transmission and reception to occur simultaneously at both stations, thus requiring clear channel operation on two different transmit frequencies at the same time. Moreover, the range measuring circuits in such systems are started and stopped by reference marker clock signals which are transmitted from each station. The range calculation is dependent upon the reference clocks being synchronized or locked to each other, and significant range errors will be produced if the clocks are not maintained in close synchronism.